I have paid thousands of pounds to lawyers on this matter. Its very simple ... sort of. Everything is ok in public until someone complains or gets the 'system' involved.
As a rule (it can be broken), a subject in public has no real redress in law if 'they are not the main subject/focus" of the shot.
Taking photos of the high street with you walking past will fall flat in court. However the frame being filled by you and only you and that picture apperaing on my web site will be making a judge ponder. Me making money from the photo will have the judge looking at motive, commercial law and all sorts.
The nighmare is that there are no real rules about this until a judge tells you that you are wrong ... or right.
As we are seeing with several issues in the press, nobody quite knows whats going on. its all a bit wikileaks.
So in my book .... you have many choices but it boils down to this. Take the shot in public and understand that this may come get you later on. Ask permission ... they may say no. Publish a photo and they may object. Sell the photo and they want their cut or sue.
Just like being a pilot, its not as easy as just flying the plane. Are you legal, within hours and up to date on air law.
In reality only test cases will set the line in the sand ....... until the tide washes it away again.
The world of 'no win no fee' will soon bring this to the news and courts. Thats when the fun starts.
The sad thing ... as i have said before ... is that in hundreds of years when people look back on archives of our time they will say this
"where are the photos of kids playing, every day street life and people" then they will say "why are there so many photos of teen girls with their boobs out"
Thinks twice press once